It will be apparent from a reading of the specification that the present invention may be advantageously utilized with fluid couplings intended for many different applications. However, the invention is especially useful when applied to a viscous fluid coupling or clutch which serves as a drive for the radiator cooling fan of a vehicle engine, and will be described in connection therewith.
Viscous fluid couplings have received wide acceptance in the automobile industry for controlling the amount of torque transmitted to a radiator cooling fan. The most common form of such viscous fluid couplings is the air temperature response type such as illustrated in U.S. Pat. Nos. 3,055,473 to Oldberg et al. and 3,809,197 to Clancey.
A disadvantage common to such viscous fluid couplings is "morning sickness" i.e., the tendency of the viscous fluid to migrate backward through the centrifugal pump into the operating chamber when the engine is shut down, causing annoying high speed operation of the fan when the engine is next started in the cold condition as in the morning. The problem of high speed operation of a viscous fluid coupling upon cold start has been partially alleviated by the use of high capacity pumps which quickly pump any fluid which has drained back into the operating chamber when the engine was off into a fluid storage chamber. Although this has reduced the duration of "morning sickness", it has not eliminated it. Additionally, a secondary problem caused with the use of high capacity pumps is that for some modes of operation, fluid tends to be pumped into the storage chamber more quickly than it can be communicated back into the operating chamber. This causes an unpredictable response time and in some cases where the input speed is very high relative to fan speed, the fan drive will not engage at all.